Radioisotopes find applications various fields such as industry, research, agriculture and medicine. Artificial radioisotopes are typically produced by exposing a suitable target material to neutron flux in a cyclotron or in a nuclear research reactor for an appropriate time. Irradiation sites in nuclear research reactors are expensive and will become even more scarce in future due to the age-related shut-down of reactors.
EP 2 093 773 A2 is directed to a method of producing radioisotopes using the instrumentation tubes of a commercial nuclear power reactor, the method comprising: choosing at least one irradiation target with a known neutron cross-section; inserting the irradiation target into an instrumentation tube of a nuclear reactor, the instrumentation tube extending into the reactor and having an opening accessible from an exterior of the reactor, to expose the irradiation target to neutron flux encountered in the nuclear reactor when operating, the irradiation target substantially converting to a radioisotope when exposed to a neutron flux encountered in the nuclear reactor, wherein the inserting includes positioning the irradiation target at an axial position in the instrumentation tube for an amount of time corresponding to an amount of time required to convert substantially all the irradiation target to a radioisotope at a flux level corresponding to the axial position based on an axial neutron flux profile of the operating nuclear reactor; and removing the irradiation target and produced radioisotope from the instrumentation tube.
The roughly spherical irradiation targets may be generally hollow and include a liquid, gaseous and/or solid material that converts to a useful gaseous, liquid and/or solid radioisotope. The shell surrounding the target material may have negligible physical changes when exposed to a neutron flux. Alternatively, the irradiation targets may be generally solid and fabricated from a material that converts to a useful radioisotope when exposed to neutron flux present in an operating commercial nuclear reactor.
The neutron flux density in the core of a commercial nuclear reactor is measured, inter alia, by introducing solid spherical probes of a ball measuring system into instrumentation tubes passing through the reactor core using pressurized air for driving the probes. However, up to date there are no appropriate irradiation targets available which have the mechanical and chemical stability required for being inserted into and retrieved from the instrumentation tubes of a ball measuring system, and which are able to withstand the conditions present in the nuclear reactor core.
EP1 336 596 B1 discloses a transparent sintered rare earth metal oxide body represented by the general formula R2O3 wherein R is at least one element of a group comprising Y, Dy, Ho, Er, Tm, Yb and Lu. The sintered body is prepared by providing a mixture of a binder and a high-purity rare earth metal oxide material powder having a purity of 99.9% or more, and having an Al content of 5-100 wtppm in metal weight and an Si content of 10 wtppm or less in metal weight, to prepare a molding body having a green density of 58% or more of the theoretical density. The binder is eliminated by thermal treatment, and the molding body is sintered in an hydrogen or inert gas atmosphere or in a vacuum at a temperature of between 1450° C. and 1700° C. for 0.5 hour or more. The addition of Al serves as a sintering aid and is carefully controlled so that the sintered body has a mean grain size of between 2 and 20 μm.
U.S. Pat. No. 8,679,998 B2 discloses a corrosion-resistant member for use in a semiconductor manufacturing apparatus. An Yb2O3 raw material having a purity of at least 99.9% is subjected to uniaxial pressure forming at a pressure of 200 kgf/cm2 (19.6 MPa), so as to obtain a disc-shaped compact having a diameter of about 35 mm and a thickness of about 10 mm. The compact is placed into a graphite mold for firing. Firing is performed using a hot-press method at a temperature of 1800° C. under an Ar atmosphere for at least 4 hours to obtain a corrosion-resistant member for semiconductor manufacturing apparatus. The pressure during firing is 200 kgf/cm2 (19.6 MPa). The Yb2O3 sintered body has an open porosity of 0.2%.
The above methods generally provide sintered rare earth metal oxide bodies adapted to specific applications such as corrosion-resistance or optical transparency. However, none of the sintered bodies produced by these methods has properties required for irradiation targets used for radioisotope production in commercial nuclear power reactors.
Powder agglomeration techniques are known to a person skilled in the art for producing compacted spherical bodies. The review article of N. Clausen, G. Petzow, “Kugelherstellung durch Pulveragglomeration”, Z. f. Werkstofftechnik 3 (1973), pp. 148-156, discloses standard agglomeration methods and the relevant physical parameters. A rotating drum for powder agglomeration is disclosed, for example, in EP 0 887 102 A2.